The present invention pertains to a method and apparatus for reducing the level of at least a portion of a signal""s frequency spectrum. More particularly, the present invention pertains to a method and apparatus for enhancing/intensifying the effect of at least one frequency band which has previously been chosen for a reduction in level.
A category of processors for altering the level of selected portions of an input signal that may have been converted to electronic analog or digital signals is known in the art. When dealing with signals in the xe2x80x9caudioxe2x80x9d frequency range (approximately between 20 Hz-20 kHz) for data such as music (or other audio sound), the category includes devices known as filters, equalizers, and tone controls.
FILTER SHAPES AND BANDS (STOP, PASS, and TRANSITION)xe2x80x94Filters are often descibed as having certain shapes, which consist of regions called bands. The four most common filter shapes are known as hi-pass, low-pass, band-pass, and band-reject (also called xe2x80x98notchxe2x80x99). Each of these filter shapes have areas (or bands) of the frequency spectrum that are affected (where the filter shape reduces the signal, or tries to stop it), and areas that are not affected (where the filter shape lets the signal pass through). A reduced area is called a xe2x80x9cSTOP-BANDxe2x80x9d (i.e., the signal is stopped). An untouched area is called a xe2x80x9cPASS-BANDxe2x80x9d (i.e., the signal is allowed to pass). There is always an area of transition between a passband and a neighboring stopband, herein called a xe2x80x9cTRANSITION BAND.xe2x80x9d
SOME PHASE CHARACTERISTICS OF WAVESxe2x80x94Wave energy (such as sound energy, alternating electrical signals, etc.) is often modeled as having peaks and troughs. Roughly speaking, a peak is where energy is at a maximum of pushing, a trough is at a maximum of pulling back, and in between the energy is shifting back and forth between this pushing and pulling.
Two identical (or very similar) waves combined in certain ways produce particular results. For the simple example of a pair of identical sine waves, some basic combinations include:
xe2x80x9cIN PHASExe2x80x9d combination=phase difference of 0 degrees=when their high and low points (peaks and troughs) occur at the same time. Since they are pushing and pulling in the same direction, playing the two waves together like this doubles the energy, which sounds louder than one wave alone.
xe2x80x9cOUT OF PHASExe2x80x9d combination=phase difference of 180 degrees=when the high point of one wave combines with the low point of the other. When out of phase, one is pushing while the other is pulling. Playing two identical waves together like this completely cancels the energy. The result is complete silence. BETWEEN 0 AND 180 DEGREES: Between 0 and 180 degrees, the two waves played together will sometimes push-pull together and sometimes push-pull in opposition. At a phase difference of 120 degrees, the resulting energy is the same amount as if only one wave was playing. As you choose phase relationships going from 0 to 180 degrees, the total energy of the two waves combined gets smaller. There is double energy at 0 degrees phase (in phase); it gets gradually less until it reaches single energy at 120 degrees phase, and continues to get gradually less until it reaches zero energy at 180 degrees phase (out of phase). From 180 to 360 degrees, the process reverses (gradually gets louder).
For example, in the audio engineering field, one often mixes signals, e.g. adds a bass drum microphone""s output to other microphone signals present. It is known in the art that the various distances from each microphone may cause phase differences that can, at certain frequencies, cause cancellations, and so affect the tone quality of the bass drum. Many audio mixing consoles, therefore, with switches that allow changing each microphone""s phase by 180 degrees (=xe2x80x9creversing phasexe2x80x9d), which is often helpful in correcting for these microphone placement problems.
A method and apparatus of the present invention makes constructive use of the phase canceling effect. In an embodiment of the present invention, a device can be constructed so that one or more of the post-processor stopbands are 180 degrees out of phase with the original (pre-processor) signal, while one or more of the post-processor passbands remain in phase (0 degrees) with the original pre-processor level. By mixing the above post-processor signal with the original pre-processor signal in approximately equal amounts, the in-phase passbands end up being somewhat louder (the greatest available increase with no added pre-processor gain is 2xc3x97). The out-of-phase stopbands become significantly reduced because the phase cancellation allows a (theoretically) infinite decrease. The present invention makes constructive use of this effect, and provides many benefits, which include:
an enhancement/extension/magnification of the processor""s effect, often beyond the ability of the processor itself,
an ability to provide a variable xe2x80x9cQxe2x80x9d/depth for circuits, some of which are normally difficult to do so, by allowing a variable slope/depth adjustment to a processor""s stop bands,
the possibility of providing a continuum from plain post-processor output to enhanced processing to no processing, all via a single control (it can also be automated for a particular setting, notably maximum enhancement).
An example of use with the single control embodiment is described below. Assume the processing involves two equalizers, one set to reduce frequencies below 200 Hz, the other set to reduce the frequencies between 2 kHz and 3 kHz. With the control set to include none of the pre-processor signal (fully xe2x80x98offxe2x80x99), one sets each processor element to a desired sound. As the phase cancellor control is turned to gradually add pre-processor signal, the listener notices an enhancement of the processor""s effect (due to the increasing phase cancellation in the stop-bands, as explained below). There will gradually be even less and less signal below 200 Hz, and less and less signal between 2 kHz and 3 kHz, until a maximum cut is reached where the control is set so that the pre- and post-processor signal mix is about even. As the control is used to add more pre-processor signal beyond that point, the regions of reduction become LESS reduced. Eventually, the pre-processor signal overwhelms the post-processor signal, until the signal is, in practice, the same as the original pre-processor output (an arrangement can be made to make it a purely pre-processor signal).
The single phase-canceling control as described can serve several purposes. Although one can make processors that have as much cut as that produced by the phase cancellation method, they are often difficult or expensive; for many filter types, having a variable cut like this would be very difficult to produce. Also, tuning with a processor with a very steep cut is often not intuitive, so this two step method (process, then enhance) is often easier to do (though some filter types commonly include the two-step method achieved another way). And since the cancellation occurs because of RELATIVE signal phase, a single phase-cancelling control can work to produce this cancellation with an entire group of processor elements: note that in the example above, the two equalizing processors are first tuned individually, whereas the enhancement to (and control of) both takes place simultaneously. In particular, this xe2x80x98globalxe2x80x99 nature of the present invention, the ability to control the intensity of an ENTIRE GROUP of processors with a single knob, is a previously unavailable effect that has an intuitive result and is easy to use.